HISTORICAL REVIEW 257 



a lithoi^liysa occurs fiequently, and ''at first sight il would secui tliat the 

 expansion of a huhl)le of gas vvitliin the hiva had occasioned the distention 

 or displacement of its layers ; hut a cai-eful study of portions of the rock 

 which exhihit great distortion and plication of the layers makes it evident 

 that in these cases the hollows occur beneath arches in the folds where 

 there has been a local relief or diminution of pressure, which might allow 

 the absorbed vapors to disengage themselves and to bi'ing about the con- 

 ditions M'hich produce hollow lithophysae in connection with spherulite 

 development. In other words, the arching of the layers appears to have 

 been the cause of the liberation of gases and the production of the cavity 

 beneath, and not the result of expanding gases." The observed relations 

 "leave no doubt that the spherulites and lithophysa% in all their com- 

 plexity of form and structure, are of primary crystallization out of a 

 molten glass, which was gradually cooling and consolidating, and that, 

 since its solidification, no alteration, chemical or mechanical, has taken 

 place/' 



The work of Tddings on the Obsidian Cliff spherulites was so thorough 

 and convincing that his conclusions ha\t' since l)c('ii Mcccptcd and applied 

 without resei've to all lithophysa'. In one particnlai', however, this gen- 

 eralization of the conclusions which hold primarily for the Obsidian Cliff 

 rocks may not be warranted, namely, that in the formation of the cavities 

 the expansion oi' the liberated gas plays no significant role. For the 

 Obsidian Cliff lithophysa^ the evidence proljably justilie(| the position 

 taken by Iddings, that the cavities were formed 1)\ a kind of uniform 

 tension in the viscous, cooling, and contracting magma (just as joints 

 are formed in a later stage of cooling), and that at such ])oints crys- 

 tallization liegjin and was accom])anied by escape of gas into the cavity. 

 I)ut it is also ))()Ssible that in other localities, as a lesult of slightly 

 dilTei-ent conditions, the pi-essui'e of the escaping gas was a factor not 

 only in enlarging the cavity, t)ut also in its initial rormation. We have 

 thus two ditt'erent hypotheses available: at the one exlrenie we lind the 

 total effect ascribed to hydrostatic tension or uniform pull developed by 

 the shrinking of the magma during cooling: at the other, to the pressure 

 of the gases set fi'ee on crystallization of the spherulites. In most cases 

 it is pidhahle that both factors, contraction of the cooling magma and 

 gas pres-iure, have been active. The primary purpose of the ])resent 

 paper is to piesent e\ idence that in the case of the Icelandic lithophySiV 

 the pri'<surc of the liberated gas was an important factor in the deveIo[)- 

 meiil (if the ca\ities. Incidentally the oiigin of certain etched surfaces 

 ol ol)>idian which i'e<ciuhle mol(la\ilic markiiiLis will he cmisiilered. 



